1. Field of the Invention
This invention relates to a photoelectric conversion device used in information processing devices such as digital still cameras, digital video cameras and image scanners. More particularly, it relates to a photoelectric conversion device having a peripheral circuit for reading electrical charges created in a light-receiving part, and a process for its fabrication.
2. Related Background Art
Photoelectric conversion devices which convert object images into electric signals include semiconductor devices of a CCD (charge-coupled device) type or an MOS (metal-oxide semiconductor) type. Recently, many have sought to improve the image-reading speed of photoelectric conversion devices. As a means therefor, peripheral circuits are made high-speed, or various peripheral circuits are integrated on the same chip as that of a photoelectric conversion section. In particular, in MOS-type photoelectric conversion devices, photoelectric conversion sections and peripheral circuit sections can be formed by a common fabrication process and hence both can be integrated on the same chip with ease. As the MOS-type photoelectric conversion devices, U.S. Pat. Nos. 5,698,844; 5,861,620; 5,955,753 and others disclose devices constituted of a light-receiving section and a signal-processing MOS transistor.
FIG. 12 is a circuit diagram of a conventional MOS-type photoelectric conversion device. In FIG. 12, reference numeral 1 denotes a diode as a light-receiving element; 2, a charge transfer MOS transistor for transferring electrical charges created in the light-receiving element 1; 3, a reset MOS transistor which, upon application of a reset pulse to a gate electrode, performs reset action on the electrical charges transferred. Reference numeral 4 denotes a selecting MOS transistor which applies a selection pulse to the gate electrode to make the circuit on; and 5, an amplifying MOS transistor which amplifies the electrical charges transferred to a floating gate. Signal charges are once stored in a storage capacitor 7 from a source follower consisting of the transistors 4 and 5 via a charge transfer MOS transistor 8 which performs transfer action according to transfer pulses, and thereafter the electrical charges of the storage capacitor 7 are outputted from an output terminal 10 as signals amplified through an amplifier 9.
FIG. 13 shows a cross-sectional structure of a photoelectric conversion section 41 having the light-receiving element 1, the charge transfer MOS transistor 2 and the reset MOS transistor 3, and a cross-sectional structure of an MOS transistor constituting a peripheral circuit section 42. Reference numeral 21 denotes a semiconductor substrate; 22, a well having a conductivity-type different from that of the semiconductor substrate 21; 23, an isolation region formed by selective oxidation called LOCOS (local oxidation of silicon); 25, a semiconductor diffusion layer serving as the light-receiving part of the light-receiving element 1; and 38, an insulating layer. Reference numerals 32 and 33 denote a source and a drain, respectively, and 31, a gate electrode. Contact holes to be formed in the isolation region 38 and electrodes formed therein are omitted in the illustration.
In logic circuits making use of MOS transistors, a salicide structure (self-aligned silicidation) is employed in which a semiconductor compound of a high-melting point metal is selectively formed on the gate electrode.
An MOS transistor having this salicide structure is cross-sectionally shown in FIG. 14.
As shown in FIG. 14, a well 122 is formed on a semiconductor substrate 121, a gate 131, a drain 132 and a source 133 are formed, and silicide layers 130 formed of, e.g., a cobalt silicide are formed on their top surfaces.
Then, on this MOS transistor, an insulating protective layer 138 is overlaid, and a gate electrode 136, a source electrode 137 and a drain electrode 135 are formed, thus a logic circuit is set up.
A solid-state imaging device provided with a silicide film only on the source and drain of a CMD device (charge modulation device) is also disclosed in Japanese Patent Application Laid-Open No. 6-326289.
In the CMD device, however, the light is received through a gate electrode formed of polycrystalline silicon and the electrical charges created by light in the underlying channel region are stored, and hence the device has been unsatisfactory in respect of sensitivity.
Meanwhile, where the salicide structure as shown in FIG. 14 is employed in the MOS-type photoelectric conversion devices for the purpose of high-speed operation, electric currents may leak from the photoelectric conversion section in a large quantity to cause deterioration of photoelectric conversion characteristics.